Porphyrins, chlorophylls, and bacteriochlorophylls are complex aromatic macrocycles which are of great biological significance, and also have novel and theoretically interesting mass spectra. In the situation where only small amounts of natural tetrapyrroles are available, mass spectrometry is virtually the only technique which can be used to obtain structural data on our materials. Primary information from mass spectrometry affords information on the various substituents on the macro-ring because the ring does not normally fragment. Mass spectra also help to establish the structures of unknown (synthetic or biological) porphyrins and chlorophylls be they unexpected reaction products or previously unknown natural substances. Our group in UC Davis focusses its efforts on synthetic and biosynthetic studies of porphyrins and chlorin systems which are the prosthetic groups in hemoglobins, myoglobins, cytochromes, catalases, peroxidases, chlorophylls, and bacteriochlorophylls. We are also heavily involved in the characterization and use of naturally derived photosensitizers for use in photodynamic therapy of malignant tumors. Mass spectrometry in these tetrapyrrole systems has beenshown, over the years, to be a vitally important component of the spectroscopic arsenal for structural investigation and chemical reactivity.